Fuel cells electrochemically convert fuels and oxidants to electricity, and fuel cells can be categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid, or solid polymer) used to accommodate ion transfer during operation. Moreover, fuel cell assemblies can be employed in many (e.g., automotive to aerospace to industrial) environments, for multiple applications.
A Proton Exchange Membrane (hereinafter "PEM") fuel cell converts the chemical energy of fuels such as hydrogen and oxidants such as air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (i.e., H.sup.+ ions) from the "anode" side of a fuel cell to the "cathode" side of the fuel cell while preventing passage therethrough of reactant fluids (e.g., hydrogen and air/oxygen gases). Some artisans consider the acronym "PEM" to represent "Polymer Electrolyte Membrane." The direction, from anode to cathode, of flow of protons serves as the basis for labeling an "anode" side and a "cathode" side of every layer in the fuel cell, and in the fuel cell assembly or stack.
Usually, an individual PEM-type fuel cell has multiple, generally transversely extending layers assembled in a longitudinal direction. In the typical fuel cell assembly or stack, all layers which extend to the periphery of the fuel cells have holes therethrough for alignment and formation of fluid manifolds that generally service fluids for the stack. As is known in the art, some of the fluid manifolds distribute fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) to, and remove unused fuel and oxidant as well as product water from, fluid flow plates which serve as flow field plates of each fuel cell. Also, other fluid manifolds circulate coolant (e.g., water) for cooling.
As is known in the art, the PEM can work more effectively if it is wet. Conversely, once any area of the PEM dries out, the fuel cell does not generate any product water in that area because the electrochemical reaction there stops. Undesirably, this drying out can progressively march across the PEM until the fuel cell fails completely. So, the fuel and oxidant fed to each fuel cell are usually humidified. Furthermore, a cooling mechanism is commonly employed for removal of heat generated during operation of the fuel cells.
Flow field plates are commonly produced by any of a variety of processes. One plate construction technique, which may be referred to as "monolithic" style, compresses carbon powder into a coherent mass. Next, the coherent mass is subjected to high temperature processes which bind the carbon particles together, and convert a portion of the mass into graphite for improved electrical conductivity. Then, the mass is cut into slices, which are formed into the flow field plates. Usually, each flow field plate is subjected to a sealing process (e.g., resin impregnation) in order to decrease gas permeation therethrough and reduce the risk of uncontrolled reactions. Typically, flow field channels are engraved or milled into a face of the rigid, resin-impregnated graphite plate. Undesirably, permeability of the graphite and machining processes therefor limit reduction of plate thickness. So, one is disadvantageously limited from increasing the number of corresponding fuel cells which occupy a particular volume in a fuel cell stack, and which can contribute to overall power (voltage, current) generation. Moreover, resin-impregnated graphite plates are susceptible to brittle failure and expensive in terms of cost of raw materials, as well as time for processing and tool wear in machining.
Another known configuration for a flow field plate embosses at least one flow field channel into a laminated assembly of compressible, electrically conductive sheets (i.e., graphite foil). The flow field plate has two outer layers of compressible, conductive sheet material and a center metal sheet interposed therebetween. The exterior surfaces of each of the two compressible outer layers constitute two major faces for the flow field plate. A flow field channel is embossed into at least one of these major faces. Such a design is disclosed in U.S. Pat. No. 5,527,363 to Wilkinson et al. (entitled "Method of Fabricating an Embossed Fluid Flow Field Plate," issued Jun. 18, 1996, and assigned to Ballard Power Systems Incorporated and Daimler-Benz AG) and U.S. Pat. No. 5,521,018 to Wilkinson et al. (entitled "Embossed Fluid Flow Field Plate for Electrochemical Fuel Cells," issued May 28, 1996, and assigned to Ballard Power Systems Incorporated and Daimler-Benz AG). A shortcoming of this design is the lack of a stable, integrated, metallic conduction path between the major faces. A further shortcoming is the inability to withstand extra compression without deformation, degradation, or flattening of the flow channel.
Thus, a need exists for easy formation of a fluid flow plate having enhanced toughness and satisfactory conductivity, strength, and sealing properties. A further need exists for such a fluid flow plate in which flow channels thereof can withstand increased compressive force in a fuel cell stack. In particular, a need exists for the fluid flow plate to maintain its shape so greater compressive loads can be applied to the fuel cell stack to compress gas diffusion layers adjacent to the plate, for advantageous savings of space.